As studies on light having a desirable frequency, those of light sources in terahertz regions in particular have actively been under way in recent years. Terahertz waves are considered applicable to various physical property measurements utilizing spectroscopy and imaging techniques employing characteristics of the terahertz waves, and their field of application has been extending over a very wide range of industry, medicine, agriculture, biotechnology, security, and the like. Therefore, much attention has been focused on how to improve techniques for generating the terahertz waves. For example, as methods for generating terahertz waves by using nonlinear optical crystals, various methods such as those utilizing birefringence (see, for example, the following Non Patent Literature 1), quasi-phase matching (see, for example, the following Non Patent Literature 2), double resonator (see, for example, the following Non Patent Literature 3), and parametric oscillation have been proposed.